This invention relates to a bipolar transistor, more particularly a power bipolar transistor.
With recent trend of rationalization and automation of various industrial installations, electronization thereof has been enhanced.
In power supplies of an electronic computer, and in such control apparatus as a motor control device utilized for a forklift, an electric motor car, etc., electric control apparatus of industrial apparatus, driving apparatus for fans and compressors of air conditioners, a control device of a numerically controlled working tool, which are required to control a large electric power generally utilize thyristors since thyristors for large power control can be easily available.
Due to the operating characteristics, the drive voltage drop in the thyristor at the time of ON-state is about 0.7 V to 1.5 V which is higher than that in a transistor, and once it becomes ON it is impossible to render it OFF unless its anode voltage reaches zero. Although OFF control is made possible by using an complicated and expensive control circuit, such circuit accompanies troubles.
A bipolar transistor requires much finer manufacturing technique than a thyristor, and even when a transistor having the same element area as that of a thyristor can be obtained the effective area for current conduction is 1/3 to 1/5 of that of the thyristor so that the heat generated by the transistor increases which renders it unsuitable for large power.
On the other hand, the voltage drop in a transistor at ON-state is 0.5 V or less which is smaller than 0.7 V to 1.5 V of that of the thyristor, and the transistor can readily be turned OFF by applying a negative bias across its base and emitter electrodes. Moreover, when compared with the thyristor the transistor has advantageous feature of high speed operation. Therefore, we have made exhaustive research to obtain a power transistor which has the feature and can control comparable power as the thyristor, and succeeded in obtaining a transistor of this invention.